## What
Reworks `stats:experiment-funnel` from a pure conversion funnel into a
**contribution-margin** one, so the 3-way trial/pricing experiment can
be judged on margin — not just conversion. Signups aren't free: each
bank connection costs money per month, so trials that connect banks and
never pay are burned cash, which the old report was blind to.
## Funnel
Per variant: **assigned → activated → carded → net-paying**
- **activated** = connected ≥1 bank connection **OR** gave AI consent
(triggered paid infrastructure), whether or not they paid.
- **carded** = completed Stripe Checkout (card on file).
- **net-paying** = live, non-refunded, mature subscription.
The **activated → carded** gap is where a user connects a bank and walks
away without paying — the exact leak the report now puts a number on.
## Metrics
| Column | Meaning |
|---|---|
| `A2P%` | net-paying ÷ activated (mature) |
| `Cost` | mature-cohort connections × cost/connection |
| `Burn` | connection cost of mature non-payers (money lost) |
| `CM` | MRR − Cost (the decision metric) |
- New `--cost-per-connection` option, default **0.40** per connection.
- Connection count includes soft-deleted/revoked connections (they still
cost money).
- All money/rate columns are gated on each variant's maturity window;
`Cost`/`Burn`/`CM` print `—` until a variant has mature volume. Legend
notes that raw `Assg`/`Actd`/`Card` are lifetime counts while
rates/money are mature-cohort only.
- Existing MRR/ARPU/Net% fields stay on the collector; dropped from the
printed table.
## Tests
Added coverage for activation (bank OR AI),
cost/burn/contribution-margin math, and the cost-per-connection
argument. Full file green (12 passed).
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